Did Google change the PageRank algorithm?

Just a few days ago, we discussed whether selling links would affect the PR value, and today comes the news that the PageRank of several world-top Blogs has been reduced.

The PR values of the famous blogs Problogger and Copyblogger both dropped from 6 to 4. Additionally, other top Blogs also experienced reductions in their PR values; for instance, Engadget's PR value dropped from 7 to 4. Perhaps this would make people associate it with the matter of selling paid links. However, neither Problogger, Copyblogger, nor Engadget have sold paid links on their blogs, so why did their PR values drop?

Could it be that Google has changed the PageRank algorithm? It remains unknown. So far, there has been no relevant statement from Google's official side.

I will continue to closely monitor the situation and provide corresponding reports. Below is a list of some websites whose PR values were reduced.